G protein-coupled receptors (GPCRs) are important in biomedicine because they are the targets of about 50% of commercially available drugs. Only little is known about the structure and function of these receptors in any molecular or atomic detail. The only three-dimensional X-ray crystallographic model for a GPCR is that of ground-state rhodopsin in an inactive conformation. Rhodopsin, the light-sensing protein in retina, has been extensively studied as a prototypical GPCR. The integral membrane protein contains seven trans- membrane helices that provide a binding site for its 11-cis-retinal chromophore. Absorption of a photon triggers a change in the chromophore's conformation and then in the receptor's tertiary structure. This results in alterations on the receptor's cytoplasmic surface that permit binding of transducin (Gt), its cognate G- protein. This initiates further steps in the signal transduction process. Most GPCRs respond to molecular signals in the form of ligands. Binding of specific ligands by specific GPCRs results in a ligand-specific cellular response. The ligand binding site for most GPCRs coincides with the retinal pocket in rhodopsin. Binding of a ligand causes the same kinds of conformational changes as does absorption of a photon in rhodopsin, and the remaining molecular mechanisms for signal transduction are similar for all GPCRs. Our understanding of GPCR structure and function will be increased by components of this proposal. First, the oligomeric state of activated GPCRS will be addressed by experiments probing the quaternary structure of rhodopsin isolated under varying detergent condtions. Assessment of physiological function will be made for these preparations. The second part of the project will use single-molecule force microscopy to probe the interactions between the membrane and rhodopsin or the serotonin 5HT1AR receptor to understand the dynamics and stabilities of GPCRs. Crystallographic studies of activated rhodopsin make up the third part of the project. The last component of the project calls for further efforts in purifying the transducin/rhodopsin complex for biochemical and structural characterization. These projects all provide structural information for an important class of proteins, a class that provides extensive experimental and theoretical challenges. The importance of the protein family for human health makes this effort worthwhile.